The present invention relates to optical devices.
Devices have been proposed in which their absorptivity for incident radiation can be controlled by the potential applied to the device. In 1973, it was proposed in UK patent No. 1 331 228 that the Pockels effect (the linear change of refractive index of some materials with field strength) could be used for modulating a light source from a laser.
Silica optical fibres as produced in recent years for optical communications have absorption minima at 1.3 and 1.55 .mu.m and therefore there is a need for devices capable of operating at such wavelengths. (The above wavelengths are in vacuo wavelengths as are all wavelengths herein except where otherwise specifically stated.)
D S Chemla, T C Damen, D A B Miller, A C Gossard, and W Wiegmann have reported, in Appl. Phys. Lett., 42(10), pages 864-866 (1983), that the absorption of photons at room temperature in a multi-quantum well structure comprising alternate GaAs and Ga.sub.0.72 Al.sub.0.28 As layers is dependent on the electric field applied in the plane of the layers of the structure. This effect they ascribe principally to the shift of exciton resonances by the Stark effect. (An exciton is a hole in the valence band in combination with an electron in the conduction band.) They report marked dependence of absorptivity with applied potential at photon energies around 1.45 eV corresponding to a wavelength of about 0.85 .mu.m (850 nm). They suggest that the effect could be used for high-speed optical modulators. One disadvantage of their proposal is that the application of a field in the plane of the layers can pose difficulties if such a device is to be integrated with others.
E E Mendez, G Bastard, L L Chang, L Esaki, H Morkoc, and R Fischer have reported, in Physica, 117B and 118B, pages 711-713 (1983), that the photoluminescence spectrum of a multi-quantum well structure at 6 K (-267.degree. C.) comprising alternate GaAs and Ga.sub.1-x Al.sub.x As layers is dependent on the electric field applied perpendicular to the layers of the structure. A complex variation is observed, both the relative height of two luminescence peaks and the positions of these peaks depending on the electric field. The authors interpret their results principally in terms of the Stark effect on excitons. The variation in question is at wavelengths around 0.75 .mu.m (750 nm) corresponding to photon energies of about 1.65 eV. Mendez et al make no reference to possible practical applications of their results, and in any case the complexity of the variation and also the low temperatures used make such application unlikely.